This application describes and claims certain improvements in the basic electrochemical cell disclosed in U.S. Pat. No. 3,791,871 issued Feb. 12, 1974.
The basic mechanism of operation of the cell described in the aforementioned patent is incorporated by reference in this application. Briefly, the cell utilizes a reactive metal anode highly reactive with water and spaced from a cathode by an electrically insulating film formed on the anode in the presence of water. The anode and cathode are immersed in aqueous electrolyte. In the embodiment shown in the aforementioned patent, the anode is formed of an alkali metal such as sodium or lithium and, during operation of the cell, the electrolyte is a liquid solution in water of an alkali metal hydroxide. Alloys and compounds of the alkali metals and other reactive metals should be equally feasible for use as the anode, however, provided they are substantially as reactive with water as are sodium and lithium and further provided, in common with sodium and lithium, they form an insulating film in the presence of water. The electrolyte is preferably an alkali metal hydroxide of the alkali metal utilized as the anode since such hydroxide is naturally formed during operation of the cell and hence automatically regenerates the electrolyte during operation. However, other alkaline electrolytes can be used to initially start up the cell or even during operation of the cell provided they permit the required anode-cathode reactions. Illustratively, potassium and ammonia hydroxide and alkali metal halides are feasible. After start-up, these electrolytes will become replaced by the hydroxide of the anode metal unless subsequent additions of these electrolytes are made during operation of the cell.
Due to anode sensitivity, there are a number of limitations to the basic electrochemical cell. It is difficult to control the reactive metal-water parasitic reaction. This further reduces the energy density in the cell and results in additional heating. One of the best methods of suppressing the parasitic reaction is to operate the cell in high molarity electrolytes. However, power from the cell appreciably decreases in high molarity electrolytes and cell performance is quite sensitive to minute changes (.about.0.01M) in, for example, lithium hydroxide concentration. Large amounts of diluent water are therefore required to control molarity and replace the water consumed by the useful electrochemical reaction.
Performance of the cell also undesirably fluctuates with changes in electrolyte flow rates, with higher power densities being realized at high pumping speeds. Desirably, high power densities should be achieved at low pumping speeds in order to reduce the power requirements for the pump. Further, it is also desirable to have power remain constant over a broad range of flow rates so as to simplify multicell battery design.
Performance of the cell is also quite sensitive to temperature, and heat exchanger requirements for the cell are substantial due to the narrow temperature operating range for efficient reactive metal anode operation.